1. Field of the Invention
The present invention relates to a Q-switched device and to Q-switched lasers, such as a scalable side-pumped laser for lidar, medical and other applications. The proposed device can enable cost-effective near single-mode operation with a smooth temporal profile and a slightly chirped spectral profile.
2. Description of the Related Art
Siegman explained in his monograph entitled Lasers (Anthony E. Siegman, Stanford University, Lasers, University Science Books, 1986, ISBN 0-935702-11-3) that Q-switched (QS) lasers are more likely to oscillate in several axial and/or transverse modes than are continuous-wave (cw) or long-pulse lasers. The details of laser dynamics are further discussed later in this document, but the oscillation essentially results from the generation of a few laser cavity modes with closely spaced respectively frequencies. Modes usually compete for the high gain during the pulse of a typical QS laser. The interference between these modes, also referred to as mode-beating, results in spikes, or intensity modulations at frequencies higher than the pulse envelope. These spikes are detrimental to the long-term reliability of practical lasers (output above a few watts), because they lead to a lower damage threshold and/or reduced lifetime for optical devices downstream of the optical train, such as nonlinear optical devices or fiber optic delivery systems. Moreover, these spikes can be highly problematic for numerous applications such as limiting the lifetime or efficiency of high power nonlinear optics conversion processes, limiting resolution or noise of lidar metrology, or limiting narrow-band laser applications such as ranged spectroscopic species identification.
Known systems (e.g., U.S. Pat. No. 4,455,657 to Robert. L. Byer) address the problem of controlling QS pulsed laser and single-longitudinal-mode operation to minimize the mode-beating noise effect. These known systems are based on an external seed-injection laser. The significance of the proposed technique is best illustrated by comparison to this known technique.
The seed-injection technique is based on the use of a single-longitudinal mode laser (the seed laser), maintained at a fixed frequency, and injected in the main QS oscillator cavity to dominate the initial stimulated emission process during the pulse build-up, as opposed to leaving random spontaneous emission to create the signal by default. The method works well, but is relatively cumbersome, expensive, and the alignment is sensitive and it is labor-intensive. Moreover, electronic control feedback is required to stabilize the oscillator cavity length, and to match the frequency of the seed laser.
By contrast, the novel Q-switch device described herein requires no additional external feedback, and does not add significant complexity to the usual QS laser cavity. Its implementation does not add significant cost to a regular QS laser to produce narrow-band spectral emission via operation with single or near-single longitudinal mode (SLM). With proper precautions in the design and implementation of the invention, an economical QS laser with single-transverse (TEM00) can be produced.